The control of the circulation of drilling fluids during hydrocarbon well drilling operations has presented a number of unique flow control problems. The presence of the earth cuttings and other highly abrasive solids in the drilling fluids have frequently interfered with or damaged the valve sufficiently to block operating movement of the flow closure elements. The presence in the drilling fluid of corrosive fluids that are encountered in the earth formations during drilling has in many instances resulted in sufficient corrosion of the valve working parts to render the valve ineffective. The valves were also extremely difficult to operate at the high working pressures unless a complicated arrangement for balancing urging of operating fluid pressure on the movable flow control components was provided. In addition, the high operating pressure of the drilling fluid often resulted in the restricted or throttled flow eroding or cutting the flow control components such as valve seats and can in time create a leakage flow path in the valve body itself.
Many drilling choke valves of rugged and simple construction and design have been developed in the past for use during drilling operations. Most drilling choke valves have also been made relatively easy to repair or otherwise maintain in the field. Despite such ease of maintenance, there has remained a need for a drilling choke valve that will indicate a failure condition is about to occur in order that preventative-type attention will be performed prior to failure. As valve failure would ordinarily result in loss of drilling fluid control and could ultimately result in loss of the well itself, such an indication of an incipient failure condition is a highly desirable valve characteristic or feature.
A flow control valve for similar usage is disclosed in copending application of John D. Muchow and Harry R. Cove, entitled "Valve Apparatus", Ser. No. 212,822, filed Dec. 4, 1980, and now abandoned, which is also assigned to the assignee of the present invention.